Can't do it the same way.
You can program a trunk directly on a key but you can't use it like you'd want to, such a fwd to vm or xfer.
What we normally do is program up keys on a phone as extension, put them in a hunt group and then program the trunks to ring the hunt group. This simulates the key system approach yet still gives all the features of a PBX.
You've got a bit of a learning curve here. Good luck with the project.
Ralph